The present invention relates generally to oscillating wave engines and refrigerators, and, more particularly, to Stirling engines, Stirling refrigerators, orifice pulse tube refrigerators, thermoacoustic engines, thermoacoustic refrigerators, and hybrids and combinations thereof.
Historically, Stirling""s hot-air engine of the early 19th century was the first heat engine to use oscillating pressure and oscillating volume flow rate in a working gas in a sealed system, although the time-averaged product thereof was not called acoustic power. Since then, a variety of related engines and refrigerators have been developed, including Stirling refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Combinations thereof, such as the Vuilleumier refrigerator and the thermoacoustically driven orifice pulse tube refrigerator, have provided heat-driven refrigeration.
Much of the evolution of this entire family of acoustic-power thermodynamic technologies has been driven by the search for higher efficiencies, greater reliabilities, and lower fabrication costs. FIGS. 1, 2, and 3 show some prior art engine examples in which simplicity, reliability, and low fabrication costs have been achieved by the elimination of moving parts, especially elimination of moving parts at temperatures other than ambient temperature.
FIG. 1 shows a free-piston Stirling engine 10 integrated with a linear alternator 12 to form a heat-driven electric generator. High-temperature heat, such as from a flame or from nuclear fuel, is added to the engine at hot heat exchanger 14, ambient-temperature waste heat is removed from the engine at ambient heat exchanger 16, and oscillations of working gas 18, piston 22, and displacer 24 are thereby encouraged. The oscillations of piston 22 cause permanent magnet 26 to oscillate through wire coil 28, thereby generating electrical power, which is removed from the engine to be used elsewhere.
The conversion of heat to acoustic power occurs in regenerator 32, which is a solid matrix smoothly spanning the temperature difference between hot heat exchanger 14 and ambient heat exchanger 16 and containing small pores through which working gas 18 oscillates. The pores must be small enough that working gas 18 in the pores is in is excellent local thermal contact with the solid matrix. Proper design of the dynamics of moving piston 22 and displacer 24, their gas springs 34/36, and working gas 18 throughout the system causes the working gas in the pores of regenerator 32 to move toward hot heat exchanger 14 while the pressure is high and toward ambient heat exchanger 16 while the pressure is low. The oscillating thermal expansion and contraction of the working gas in regenerator 32, attending its oscillating motion along the temperature gradient in the pores, is therefore temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low.
The absence of crankshafts and connecting rods contributes to the simplicity, reliability, and low fabrication costs of the free-piston Stirling engine.
FIG. 2 shows a xe2x80x9ctoroidalxe2x80x9d regenerator-based engine: a thermoacoustic-Stirling hybrid engine delivering acoustic power to an unspecified load 42 (e.g., a linear alternator or any of the aforementioned refrigerators) to the right. See, e.g., U.S. Pat. No. 6,032,464, xe2x80x9cTraveling Wave Device with Mass Flux Suppression, issued Mar. 7, 2000, to Swift et al. and U.S. Pat. No. 6,314,740, xe2x80x9cThermoacoustic System,xe2x80x9d issued Nov. 13, 2001, to deBlok et al. High-temperature heat, such as from a flame, from nuclear fuel, or from ohmic heating, is added to the engine at hot heat exchanger 44, most of the ambient-temperature waste heat is removed from the engine at main ambient heat exchanger 46, and oscillations of the working gas are thereby encouraged. Mass flux suppressor 50 acts to minimize time-averaged mass flux of the working gas and attendant heat loss. The oscillations deliver acoustic power to load 42.
FIG. 3 shows a xe2x80x9ccascadexe2x80x9d thermoacoustic-Stirling hybrid engine comprising a standing-wave thermoacoustic engine and a Stirling engine in series, without any piston therebetween, as described in U.S. patent application Ser. No. 10/125,268 xe2x80x9cCascaded Thermoacoustic Devices,xe2x80x9d G. W. Swift et al., filed Apr. 18, 2002. High-temperature heat is added at the two hot heat exchangers 52, 54; ambient-temperature waste heat is removed at the three ambient heat exchangers 56, 58, 62; and oscillations of the working gas are thereby encouraged. The oscillations deliver acoustic power to a load 64, such as a linear alternator or a pulse tube refrigerator, below the bottom of FIG. 3. The conversion of heat to acoustic power occurs in regenerator 66 according to the same processes as described in the context of FIG. 1 above. Stack 68 has larger pore sizes than regenerator 66, and conversion of heat to acoustic power in stack 68 occurs by a similar process, but with some different details regarding time phasing, as described in the ""268 patent application.
The simplicity, reliability, and low fabrication cost of the toroidal thermoacoustic-Stirling hybrid engine and of the cascade thermoacoustic-Stirling hybrid engine, compared to earlier Stirling engines, comes from the elimination of pistons previously needed.
FIG. 4A shows a piston-driven orifice pulse tube refrigerator, as described for example by R. Radebaugh in xe2x80x9cA review of pulse tube refrigeration,xe2x80x9d Adv. Cryogenic Eng., volume 35, pages 1191-1205 (1990). The motion of piston 70 causes oscillations in the working gas throughout the refrigerator. Low-temperature heat is removed from a load by the refrigerator at cold heat exchanger 72, and ambient-temperature waste heat is rejected from the refrigerator at the two ambient-temperature heat exchangers 74, 76, the larger of which is commonly called the aftercooler, i.e., heat exchanger 74. Heat pumping up the temperature gradient occurs in regenerator 78 because the working gas in the pores of regenerator 78 is caused to move toward cold heat exchanger 72 while the pressure is high and toward aftercooler 74 while the pressure is low. This necessary time phasing between oscillating pressure and oscillating motion is created by acoustic impedance network 82 above pulse tube 84, which sets the relative amplitudes and time phasing of the pressure and velocity at its entrance. Earlier Stirling refrigerators achieved the correct time phasing by means of a cold piston (whose motion was coordinated with that of the drive piston) instead of by means of the acoustic impedance network. However, the technical challenge of sealing around such a piston at cryogenic temperatures was severe. Hence, the simplicity, reliability, and low fabrication cost of the orifice pulse tube refrigerator compared to earlier Stirling refrigerators comes from the elimination of the cold piston.
Although much progress has recently been made in the elimination of moving parts from these oscillating-wave engines and refrigerators, the simplification of the heat exchangers offers a second opportunity for dramatic improvement in simplicity, reliability, and low fabrication cost, particularly in engines and refrigerators of high power. All engines and refrigerators must reject waste heat to ambient temperature, and the ambient temperature is often present as a flowing fluid stream, such as a fan-driven air stream or a flowing water stream. Engines must also accept heat from a source at a higher temperature, which may be in the form of a flowing stream of combustion products from a burner. Refrigerators must withdraw heat from a load at lower temperature, which is sometimes in the form of a flowing stream; examples include a stream of indoor air to be cooled and dehumidified, or a stream of methane to be cooled and cryogenically liquefied. Hence, the typical heat exchanger in these engines and refrigerators must transfer heat between a steadily flowing xe2x80x9cprocess fluidxe2x80x9d stream and an oscillating xe2x80x9cworking gasxe2x80x9d stream that is the thermodynamic working substance of the oscillating-wave engine or refrigerator. The working gas is often pressurized helium gas. At small power levels, simple geometries such as stacks of copper screens suffice as heat exchangers, but at higher powers the thermal conductivity of solids is insufficient to carry the required heats, so that geometrically complicated heat exchangers must usually be used to bring the process fluid and working gas into intimate thermal contact.
M. Mitchell, xe2x80x9cPulse tube refrigerator,xe2x80x9d U.S. Pat. No. 5,966,942, Oct. 19, 1999, teaches a design to avoid a geometrically complicated heat exchanger for the ambient heat exchanger 76 (FIG. 4A) at the ambient end of the pulse tube 84 of an orifice pulse tube refrigerator. As illustrated in FIG. 4B, which is adapted from FIGS. 1 and 11 in the ""942 patent, ambient heat exchanger 76 and orifice 86 can be replaced by a dissipative heat-transfer loop 88 containing one or more (two are shown in FIG. 4B) fluidic diodes 92, 94 that convert some of the oscillatory power in the oscillating wave into circulating flow of the working gas around loop 88. The dissipation in fluidic diodes 92,94 and other oscillatory dissipation in loop 88 serve the function of orifice 86, and the surface area along the entire path length of loop 88 serves the function of heat exchanger 76.
A shell and tube heat exchanger 102, illustrated in FIGS. 5A and 5B, is typical of the complicated, geometries that must otherwise be used at high power throughout oscillating-wave engines and refrigerators. Working gas 104 oscillates through the insides of the many tubes 106, while process fluid 108 flows around and between the outsides of tubes 106.
Particular features of oscillating-wave engines and refrigerators impose size constraints on such heat exchangers as they are scaled up to higher powers. Higher power demands more heat-transfer surface area for efficient heat transfer. However, tube diameters cannot be increased, because this would reduce the total heat-transfer coefficient on the working-gas side, thereby decreasing the efficiency. Tube lengths cannot be increased, because having such tube lengths greater than the oscillatory displacement of the working gas does not help transfer more heat.
The usual solution to the scaleup of heat exchangers is to increase the number of tubes in proportion to the power, keeping the length and diameter of each tube constant. Such heat exchangers can have hundreds or thousands of tubes. Building such heat exchangers is expensive (because many parts must be handled, assembled, and joined) and such heat exchangers are unreliable (because so many joints must be leak tight). Thermally induced stress imposes an additional challenge to reliability when a geometrically complex heat exchanger is at an extreme temperature, such as a red-hot temperature for an engine or a cryogenic temperature for a refrigerator. Sometimes a pool boiler or heat pipe must be used to enforce isothermality in these circumstances so that thermally induced stresses are eliminated.
Another shortcoming of oscillating-wave engines and refrigerators is that their heat exchangers often must be located close to one another, simply because each heat exchanger must typically be adjacent to one end or the other of the nearest stack or regenerator or pulse tube or thermal buffer tube, and these components themselves are typically short. The practical importance of this shortcoming is easily appreciated by considering the food refrigerator in the typical American kitchen. The xe2x80x9cvapor compressionxe2x80x9d (also known as xe2x80x9creverse Rankinexe2x80x9d) cooling technology employed therein allows complete flexibility in the geometrical separation of the cold heat exchanger, where heat is absorbed from the inside of the cold box, and the ambient heat exchanger, where waste heat is rejected outside, to the air in the kitchen. The cold heat exchanger is typically located inside, above, or under the freezer, and the ambient heat exchanger is typically located behind or under the refrigerator cabinet. Not only can these heat exchangers be located freely, but their shapes can be chosen as needed for their circumstances, e.g., to accommodate fan-driven or natural convection as chosen, and to fit in and around the desired shape of the cold box or cabinet. In contrast, when one tries to adapt an oscillating-wave refrigerator to this application, the cold heat exchanger and main ambient heat exchanger must be very close together, separated only by the regenerator whose length is typically only a few inches. Hence, in, order to put the cold heat exchanger and the main ambient heat exchanger into thermal contact with the inside of the cold box and with the outside air, respectively, intermediate heat transfer means, such as heat pipes or pumped fluid heat transfer loops, must typically be employed. These add complexity and cost, and reduce efficiency.
Accordingly, it is highly desirable to provide simplicity, reliability, and low fabrication cost of heat exchangers for oscillating-wave engines and refrigerators. More specifically, the present invention is directed to eliminating the need for massively parallel heat-exchanger structures in oscillating-wave engines and refrigerators of high power. The present invention also allows the heat exchangers of oscillating-wave engines and refrigerators to be located distant from one another and from the nearest regenerator or stack.
Those skilled in the art understand that xe2x80x9cambientxe2x80x9d temperature, referring to the temperature at which waste heat is rejected, need not always be a temperature near ordinary room temperature. For example, a cryogenic refrigerator intended to liquefy hydrogen at 20 Kelvin might reject its waste heat to a liquid-nitrogen stream at 77 Kelvin; for the purposes of this particular cryogenic refrigerator, xe2x80x9cambientxe2x80x9d would be 77 Kelvin.
Those skilled in the art also understand that fluidic diodes are typically much less perfect than electronic diodes. Fluidic diodes usually offer a difference between forward and backward flow resistances of less than a factor of ten, and sometimes even less than a factor of two, whereas the difference in forward and backward resistances in electronic diodes is typically orders of magnitude. Fluidic diodes include the vortex diodes described in ""942, the valvular conduit described by Nikola Tesla in U.S. Pat. No. 1,329,559, Feb. 3, 1920, and the conical and tapered structures called jet pumps in many recent publications such as U.S. Pat. No. 6,032,464, xe2x80x9cTraveling Wave Device with Mass Flux Suppression,xe2x80x9d supra; S. Backhaus, et al., xe2x80x9cA thermoacoustic-Stirling heat engine: Detailed study,xe2x80x9d J. Acoust. Soc. Am., volume 107, pages 3148-3166 (2000); G. W. Swift, Thermoacoustics: A unifying perspective for some engines and refrigerators, to be published by The Acoustical Society of America, 2002.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention is directed to an oscillating-wave engine or refrigerator having a regenerator or a stack in which oscillating flow of a working gas occurs in a direction defined by an axis of a trunk of the engine or refrigerator, and having an improved heat exchanger. First and second connections branch from the trunk at locations along the axis in selected proximity to one end of the regenerator or stack, where the trunk extends in two directions from the locations of the connections. A circulating heat exchanger loop is connected to the first and second connections. At least one fluidic diode within the circulating heat exchanger loop produces a superimposed steady flow component and oscillating flow component of the working gas within the circulating heat exchanger. A local process fluid is in thermal contact with an outside portion of the circulating heat exchanger loop.